The present invention relates generally to heat sealing machines for closing and sealing packaging made from a heat-sealable film such as polyethylene, and in particular to means for the maintenance of a minimum gap between two opposing heat transfer components such as heat sealing bars between which layers of film to be sealed travel.
Sealing machines of the class to which the present invention generally pertains employ a pair of continuous bands, made of steel, fiberglass or other similar materials, which are arranged to travel in the same direction in confronting parallel relation with each other along a sealing run. The film layers of plastic material to be sealed together travel sequentially along the sealing run between the bands, and the seal is made by simultaneously applying heat and pressure to the layers as the layers travel between the bands along the sealing path. The material to be sealed is carried between a pair of continuous belts or chains also arranged to travel in the same direction in confronting parallel relation and at the speed of the sealing bands.
At least one pair of components such as bars or platens flanking the continuous bands apply heat and pressure to seal the layers through the bands. Such heater bars are normally mounted on the frame of the machine and have generally been stationary with respect to the frame of the machine, so that they remain close to the traveling bands while making allowance for the thickness of the film material to be sealed traveling therebetween. Ample room has had to be supplied so that the film material would not produce jamming of the machine in the event that wrinkles or multiple thicknesses were encountered during high speed operation.
More recently, the heater bars have been mounted on springs so that the entire heater bars are movable inwardly and outwardly a certain amount and apply a constant pressure against the moving bands, and thus the film material being heat sealed, so that the sealing is effectuated under spring loaded compression pressure. Allowance may then be made for the heater bars to separate a certain amount from each other and allow bands to separate or pivot slightly when wrinkled portions of film layers or other multiple thicknesses pass through the machine during sealing. One such system incorporating spring loaded heat transfer elements which continuously bear inward against the bands with a pressure device from springs is shown in U.S. Pat. No. 4,016,026.
In U.S. Pat. No. 5,034,087, commonly assigned with the present application and incorporated herein by reference, at least one heater bar of an opposing pair of bars is provided with a fluid-pressure adjustable, pivotable and spring-bias bar system which controls and modulates the maximum force applied to the bands between the bars in relation to the thickness or configuration of the layers to be sealed. In this manner varying thicknesses of materials can be easily accommodated.
In the prior art sealing apparatus, the movable heater bars move in an inward direction to thereby decrease the size of the gap between the sealing bands through which the material to be sealed travels. In certain instances, where material to be sealed is relatively thin, this laterally-inward bar movement may be of such a magnitude that the bar involved travels beyond the longitudinal mid-line of the gap. When this occurs, the material to be sealed can become misaligned, and jamming of material can result. This is also important when using glass cloth bands as they are not as strong and abrasion resistant as stainless steel bands. Contact between the heater bar and glass cloth bands results in rapid or premature wear of the bands. In view of this problem, it is apparent that a need exists for effective maintenance of inward travel distance of movable heater components.
It is therefore a primary object of the present invention to provide a sealing machine wherein the laterally-inward travel of movable heater components is controlled to prohibit such travel beyond the longitudinal mid-line of a gap disposed between opposing heater bars and sealing bands.
Another object of the present invention is to provide a sealing machine wherein laterally-inward travel of a movable heater component is limited by flexible control means attached to the bar such that the component can move laterally outwardly as well as inwardly, but not inwardly beyond the longitudinal mid-line of the gap.
These and other objects of the present invention will become apparent throughout the description which follows.